We shall be studying the origins and migration pathways of precursors or IgA plasma cells of mucous membranes. By means of double cell transfer in mice, we shall attempt to demonstrate that immature precursor cells in Peyer's patches that themselves do not directly home to intestinal mucosa will manifest such homing if first allowed to differentiate in mesenteric lymph nodes. Another aspect will deal with the role of disulfide interchange enzymes in the assembly of secretory IgA. Such an enzyme will be purified from human placenta and tested for its ability to promote covalent assembly of IgA and secretory component. Finally, the ability of IgA immune complexes to activate the alternative complement pathway will be studied.